Synthesis and characterization of amphiphilic polyelectrolyte brush copolymers based on Poly(2,7-carbazole)

Article abstract of DOI:10.1021/ma901020v

Amphiphilic polyelectrolyte brush polymers with a rigid, hydrophobic backbone formed by poly(2,7-carbazole) and flexible pH-sensitive side chains of poly(L-lysine) (PLL) are presented and their aggregation is studied. The hydrophilic polyelectrolyte side

Full text of DOI:10.1021/ma901020v

Macromolecules 2010, 43, 467–472 467
DOI: 10.1021/ma901020v
Synthesis and Characterization of Amphiphilic Polyelectrolyte Brush
Copolymers Based on Poly(2,7-carbazole)
€
Andrea Fruth, Markus Klapper, and Klaus Mullen*
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
Received June 26, 2009; Revised Manuscript Received November 7, 2009
ABSTRACT: Amphiphilic polyelectrolyte brush polymers with a rigid, hydrophobic backbone formed by
poly(2,7-carbazole) and flexible pH-sensitive side chains of poly(L-lysine) (PLL) are presented and their
aggregation is studied. The hydrophilic polyelectrolyte side chains of PLL are attached to the 9-position of
every second carbazole unit, alternatingly to hydrophobic alkyl side groups, substituting the residual
moieties. The PLL side chains can be switched between different conformations by varying the pH. At lower
pH the free amino groups in PLL become protonated and the side chains have a rodlike conformation, while
at higher pH values the PLL chains form in their unprotonated form coil structures. This process is proven by
circular dichroism and 13C NMR experiments. Fluorescence emission spectra, dynamic light scattering, and
atomic force microscopy (AFM) experiments demonstrate that the aggregation behavior of the synthesized
polyelectrolyte brush polymers can be controlled by the pH as well. In the charged form the polymers are
observed as single molecules due to the electrostatic repulsion; in its unprotonated form, however, large
aggregates are formed. The water-soluble brush polymers can be considered due to the stiffness of the
backbone and the switchable PLL side chains either as a rod-g-coil or a rod-g-rod polymer.
Introduction
polymer with a poly(2,7-carbazole) backbone and alternating
side chains of poly(L-lysine) (PLL) and hydrophobic 2-ethylhexyl
groups (1) (Figure 1).
Conjugated polymers are active components of electronic
devices,1 as for example in plastic solar cells,2 light-emitting
diodes,3 and field-effect transistors.4 Their electronic and optical
properties such as charge carrier mobility, light absorption, and
emission strongly depend upon the molecular structure but also
upon the morphology obtained in bulk or at interfaces after
processing.5 Amphiphilic block polymers with π-conjugated rigid
backbones became of special interest since they enable the control
of the aggregation and packing behavior in solution, in bulk, or at
interfaces.6-9 As a result, they gave rise to cylindrical, shape-
persistent micelles and show microphase separation.8,10-12 In the
case that the amphiphilicity was caused by charges as for example
for polyphenylene sulfonates,13 the aggregation properties and
the packing behavior were additionally influenced by the pH as
the amphiphilicity of the polymers was adjustable by the degree
of protonation.14,15 Remarkably, the conformation of the con-
jugated polyelectrolytes and therefore the effective conjugation
length have not been affected by the ionic strength of the applied
system.16-19 To further increase the amphiphilicity, attempts
were made to use instead of the charged groups, which are
directly attached to the backbone, polyelectrolyte side chains
and to obtain amphiphilic conjugated brush polymers. It was
expected that the side chains allow for a better control of the
morphology by self-assembly and additionally show a stimulus-
responsive behavior e.g. by varying the pH.9 In particular,
polyphenylenes with polystyrenesulfonate side chains were pro-
posed.20 This concept included the synthesis of macromonomers
from polystyrenesulfonate ethyl esters carrying polymerizable
phenylene end groups. However, the polymerization failed which
was attributed to the steric hindrance caused by the long side
chains on the reactive units of the phenylene moieties. Herein, we
present the synthesis of a conjugated polyelectrolyte brush
We selected polycarbazole for the backbone as it is one of the
most applied materials for photovoltaics and as especially for the
application in solar cells a control of the aggregation during
device formation is required.21 Synthetically this polymer is
attractive as for a carbazole building block functionalized with
PLL grafts the steric hindrance should be lower in a polycon-
densation process in comparison to the smaller phenylene units.
Additionally, carbazole allows for a specific monofunctionaliza-
tion in the 9-position, the exclusion of defect structures caused by
side reactions such as oxidation,22 and a better conjugation due to
a reduced torsion along the backbone as a result of the nitrogen-
bridged biphenyls.23 PLL homopeptide was chosen due to its pH-
responsive properties similar to polyelectrolytes and also due to
the formation of the typical secondary structures of peptides.24,25
PLL exhibits a random coil structure below pH 12, changing
spontaneously into a helical conformation with rising pH.26 The
obtained polyelectrolyte brush polymers are characterized, and
their aggregation behavior depending on pH is described.
Results and Discussion
Polymer Synthesis. To obtain the amphiphilic polymer 1,
the synthesis via a macromonomer approach,27 in a related
fashion used for PPP graft polymers,7,12,28,29 was adopted.
The polymerization employed dibromide 4 and diboronic
ester 5 of two carbazole derivatives that were subsequently
reacted via Suzuki polycondensation (Scheme 1). PLL-con-
taining macromonomers were synthesized starting from
2,7-dibromocarbazole with the 9-position functionalized
with phthalimide (2).30 Compound 2 was converted into
amine 3 in a reaction with hydrazine monohydrate.31 Mono-
disperse poly(ε-Z-L-lysine(Z))-OH was synthesized by means
of solid phase chemistry32 and coupled to the carbazole
*Corresponding author: e-mail muellen@mpip-mainz.mpg.de; Fax
(þ49)6131-379-100.
r 2009 American Chemical Society
Published on Web 12/09/2009
pubs.acs.org/Macromolecules

468 Macromolecules, Vol. 43, No. 1, 2010
Fruth et al.
Figure 1. Polyelectrolyte comb-type polymer 1 with a conjugated backbone and polyelectrolyte side chains.
Scheme 1. Synthesis of the Conjugated Polyelectrolyte 1a
derivative 3 via HOBt-activated ester synthesis to yield macro-
monomer 4. The diboronate ester with branched alkyl chain 5
was synthesized according to the literature.33 Monomers 4 and
5 were polymerized in a Pd-mediated Suzuki-type polycon-
densation to generate the uncharged precursor 6. Polymer 6
was then converted into the conjugated polyelectrolyte brush
copolymer 1 by removal of the Z-protecting group with HBr/
acetic acid (5.7 M) in trifluoroacetic acid.34 The complete
disappearance of the Z-group signals in the 1H and 13C
NMR spectra proved the quantitative cleavage of the
Z-protection group. Under these strongly acidic conditions,
the amino functionalities of PLL were transformed into
ammonium groups increasing the hydrophilicity. Since it is
known from other PLL systems that a dried sample is difficult
to be redissolved,26 our polymer was kept in aqueous solution
until further investigation.
As PLL was synthesized by the solid phase technique, it
was monodisperse to give structurally defined macromono-
mers and could thus facilitate further characterization. The
degree of polymerization (DP) of the PLL side chain was
chosen to be 20 to adopt a pH-responsive secondary struc-
ture.35 During polymerization, the amino groups of the
lysine monomers were protected by the benzoyloxycarbonyl
(Z-) group as free amines have been reported to lower the
yield in Suzuki-polycondensation reactions.36 This Z-group
is stable to most basic and acidic conditions and therefore
suitable for all reaction conditions used. The introduction of
a branched alkyl chain at the N-position in compound 5
became necessary to enhance the solubility of the resulting
polymer in common organic solvents such as DMF. During
Suzuki polymerization the temperature was decreased to
75 °C in comparison to standard conditions (T > 80 °C)37
to avoid denaturation of the PLL.
a Conditions: (i) hydrazine monohydrate, MeOH, reflux, 2 days,
97%; (ii) oligo(ε-Z-L-lysine(Z))-OH, HOBt, HBTU, DIPEA, NMP,
argon, rt, 3 days, light-exclusion, 94%; (iii) Pd(PPh3)4, DMF, 2 M
K2CO3, 75 °C, 5 days, 40-80%; (iv) HBr/acetic acid (5.7 M), TFA, rt,
1 h, 95%.

Article
Macromolecules, Vol. 43, No. 1, 2010 469
Table 1. GPC Data of Suzuki Polymerizations with Variation in Side-Chain Length of Poly(L-lysine(Z)), Determined against PS Standards in
DMF
_
_
_
M n/M w
_
DP (PLL) macromer
M n/g mol-1
M w/g mol-1
DP
monomer conversion
yield/%
6a
6b
6c
6d
3
10
15
20
1.5 ꢀ 103
19.0 ꢀ 103
15.4 ꢀ 103
58.3 ꢀ 103
3.9 ꢀ 103
29.5 ꢀ 103
26.8 ꢀ 103
103.7 ꢀ 103
3.1
1.5
1.8
1.8
2
12
9
40
75
78
65
0.83
0.75
0.90
20
Polymer aggregation is a known phenomenon of polyelec-
trolytes with hydrophobic groups even in very dilute solu-
tions.18 This effect is seen as the major drawback with regard
to precise structural characterization, such as GPC and light
scattering.38,39 Therefore, the molecular weights were mea-
sured of the neutral precursor 6. GPC analysis (DMF, poly-
(styrene) (PS) standard) revealed in the best case 6d a
_
number-average molecular weight (M n) of 58.3 ꢀ 103
g
_
mol-1, a weight-average molecular weight (M w) of 103.7 ꢀ
103 g mol-1, and a polydispersity of 1.82 (Table 1). The
obtained Mn corresponded to a degree of polymerization
(DP) of 20. The results of GPC analysis, as a relative method,
were compared to static light scattering (SLS) for absolute
Mw determination to evaluate the obtained molar mass since
no molecules were available to serve as a standard for
_
compound 6. SLS analysis detected for 6d a M w of 106 ꢀ
103 ( 5.5 ꢀ 103 g mol-1. As GPC gave results comparable to
SLS, only this very fast method GPC was used as a standard
method for the determination of the molecular weights.
The low DP of the precursor polymer 6a-6c was attrib-
uted to various reasons. First, there was a significant differ-
ence in molecular weight of the monomers used, which
Figure 2. Circular dichroism spectra of compound 1 in dependence on
the pH with cpolymer = 0.19 mg mL-1
.
under acidic and neutral conditions, minima at 195 nm and
maxima at 215 nm were found, which indicated unordered
structures. Further increasing of the pH resulted in two
minima at 205 and 222 nm. These are characteristic for
helical structures.41-43
The results demonstrated a conformational change for the
PLL side chains in polymer 1 from random coil to helix with
increasing pH. This effect corresponded to the deprotona-
tion of ammonium species in the ε-position of the lysine. The
disappearance of the repulsive force finally led to ordered
helical structures. Thus, it was proven that, beside the ionic
strength of the system, PLL side-chain conformation of
polymer 1 is also responsive to the pH in solution. This
comb-type polymer, therefore, can be considered as the first
structure not only with a rigid backbone and a polyelec-
trolyte side chain but also with a rod-g-coil structure which
can be switched to a rod-g-rod structure (Figure 1).
For confirmation of the conformational changes, the PLL
side chains were further investigated by solid-state NMR
experiments performed on the polymers 1 and 6d. 13C cross-
polarization magic-angle spinning (CPMAS) NMR spec-
troscopy was used to elucidate the conformational features
from the conformation-dependent 13C chemical shifts.44 The
conformation was determined by the chemical shift of the
PLL amid peak at δ ∼ 170-180 ppm.45 Conformational
analysis of the PLL amide peak of the solid precursor
polymer 6d gave a ratio of 82% R-helix to 18% β-sheet. This
ratio was consistent with the values obtained for unsubsti-
tuted poly(L-lysine(Z)) homopeptides.44,45 In contrast, com-
pound 1 exclusively exhibited a coil conformation. In
analogy to the liquid phase, these unordered structures were
explained by the electrostatic repulsive forces between ad-
jacent charged ammonium groups along the PLL side chains
as a consequence of the cleaving procedure of the Z-group.
To investigate the influence of the degree of protonation
on the conformation of PLL side chains in solid state, sample
1 was treated with aqueous solutions of different pH values.
After lyophilization, CPMAS-NMR analysis showed a ran-
dom coil conformation for samples obtained from the aqu-
eous solution with a pH of 2.5 (a) and 10 (b). Increasing the
caused difficulties in meeting the 1:1 stoichiometry essential
_
for achieving high M n in a polycondensation process. There-
fore, in a series of experiments the monomer amounts were
increased from 546 to 2.208 mg to ensure stoichiometric
control. This optimization resulted for 6d in a higher molar
mass, with an increase from 21.2 ꢀ 103 to 58.3 ꢀ 103 g mol-1
.
Second, the steric hindrance of the polymerizable function-
alities by the bulky poly(L-lysine(Z)) side chain in macro-
monomer 4 was proposed to limit the obtained molar mass.
Consequently, shorter PLL side chains were used. Contrary
to our expectations, substitution with shorter PLL resulted
in lower DP (Table 1, 6a, 6b, 6c vs 6d). This effect on DP was
probably caused by a precipitation40 of the formed oligomers
6a-6c during polymerization, resulting from a lower solu-
bility due to the decrease in the length of the PLL side chain.
The reduced reaction temperature during polymerization in
comparison to standard conditions is proposed as a further
reason for the lower DPs.
Monomer conversions were calculated for the obtained
DP and the values are reported in Table 1. They were found
to be higher than the yield of the polymerization reaction.
The differences between the yields and monomer conver-
sions were attributed to fractionation during the precipita-
tion process, which caused DPs lower than 2.
Conformational Characterization. Conformational Inves-
tigation of the PLL Side Chain. Further investigation on the
pH sensitivity was performed only on polymer 1 obtained
after cleavage of the protecting group from 6d as only poly-
(L-lysine(Z)) with a DP higher than 15 can adopt a pH-
responsive secondary structure.14 Circular dichroism experi-
ments were performed in order to obtain information on the
secondary structure of the PLL side chains within our comb-
type polymer 1 in dependence on the pH in solution. The
molar ellipticities θ of compound 1 for different pH are
displayed in Figure 2. The neutral precursor 6d could not be
investigated as it was only soluble in DMF, which shows an
overlapping absorption at λ < 260 nm. For compound 1

470 Macromolecules, Vol. 43, No. 1, 2010
Fruth et al.
Table 2. Hydrodynamic Radii of Compound 1
(cpolymer = 1.13 mg mL-1) for Different pH
pH
Rh/nm
0
2
7
85 ( 6
85 ( 5
94 ( 6
10
11
12
14
135 ( 9
128 ( 8
179 ( 15
111 ( 9
fluorescence showed one maximum at 425 nm and a shoulder
at 460 nm. With increasing pH the fluorescence broadened
significantly and further maxima arose at 497 and 459 nm.
No isosbestic point was monitored, which would indicate a
change between two different chromophoric species. The
changes in fluorescence of compound 1 as a function of pH
suggested that the polymer was aggregated in basic media
and less aggregated or unaggregated under acidic and neu-
tral conditions. The broad fluorescence of the uncharged
polymer at high pH was attributed to emission from an
“excimer-like” state arising from interchain π-π stacking.14
This broad emission is characteristic for conjugated poly-
mers in the solid state when interchain aggregation is occur-
ring.13 Since lysine has a pKa = 10.5, the side chain was
partially neutralized at pH between 10 and 11. Thus, electro-
static repulsion was decreased for increased pH and inter-
chain aggregation occurred.
Figure 3. Optical properties of compound 1 as a function of pH: (a)
absorption spectra; (b) emission spectra (λexc = 370 nm) in water.
In anticipation of these changes, the formation of aggregates
of compound 1 was investigated by dynamic light scattering at
different pH (Table 2). At pH e 7, the hydrodynamic radius
(Rh) was in the range of 85-94 nm. For higher pH, an increase
of the particle size was found. These data clearly indicated that
an aggregation of chains occurred upon deprotonation of the
ammonium groups in the polylysine side chains as the electro-
static repulsion became minimized.
UV-vis and PL characteristics of the brush polymer 1 in
dependence on the ionic strength in solution are displayed in
Figure 4. The ionic strength was adjusted by different con-
centrations of guanidine hydrochloride. Similar to the pH-
dependent measurements, no shift of the absorption maxima
at a wavelength of 370 nm and the emission band at 425 nm
and its shoulder at 453 nm with increasing the ionic strength
was found. While the intensity of the absorption band was
slightly reduced, the fluorescence band decreased and broa-
dened significantly. Again, the broadening of fluorescence
was attributed to the formation of aggregates which were
also found in DLS. It was assumed that, typical for poly-
electrolytes,20 the addition of salt shields the electrostatic
repulsive forces causing the interchain aggregation.
No shift of λmax and thus no conformational changes were
found in the results of UV-vis and PL characteristics of
polymer 1. This behavior confirmed that the conformation
of the π-conjugated poly(2,7-carbazole) backbone was in-
dependent of the pH and ionic strength and that conforma-
tional changes within the brush polymer were restricted to
the flexible polyelectrolyte PLL side chain.
Morphological Investigations. Initial studies on the surface
morphology of the polyelectrolyte brush polymer 1 were
performed by AFM measurements to obtain structural in-
formation at the molecular level. Mica was chosen for
polymer adsorption because its negatively charged surface
becomes a multivalent counterion for the cationic PLL
brushes.48,49 Measurements displayed untextured wormlike
particles which had a uniform height and width but a
bimodal length distribution. While they possessed a width
of around 3-5 nm and a height between 1 and 2 nm, the
pH to 14 gave a 45% R-helix and a 55% β-sheet. This
behavior was consistent with PLL adopting helical and β-
sheet secondary structures for pH > 12.26 The occurrence of
a β-sheet conformation can be explained by the sample
preparation.35,46 After freeze-drying, a change to a more
predominant β-sheet structure is known for PLL homopep-
tides in the solid state, which is energetically more favorable
in a dried solid.47 To prove the reversibility of the conforma-
tional changes, the sample 1 obtained at pH = 14 was
redispersed in a strongly acidic solution (pH = 0), and a
switch-over from R-helix/β-sheet to random coil was found.
By contrast, the transition from acidic to basic conditions
resulted in a change from random coil to a mixture of 40%
R-helix and 60% β-sheet. Hence, the conformational change
between random coil and R-helix/β-sheet was proven to be
reversible. The appearance of β-sheet conformation de-
scribed the existence of strong hydrogen bonds between the
PLL side chains.
Analysis of the Conformation of the Poly(2,7-carbazole)
Backbone. Furthermore, the effect of the conformational
changes of the stimulus-responsive side chains on the con-
formation of the poly(2,7-carbazole) backbone was investi-
gated. It was expected that any conformational changes of
the poly(2,7-carbazole) would affect its conjugation length
and could thus be monitored by absorption and fluorescence
characteristics of the polyelectrolyte brush polymer 1.
pH-dependent UV-vis and PL spectra of compound 1
were recorded in an aqueous solution at different pH values
ranging from 0 to 14 (Figure 3), normalized to the most
concentrated sample. The pH of the sample was adjusted
with HCl and NaOH. Polymer 1 showed a strong absorption
band at a wavelength of 370 nm, which arose from the π, π*
transition. Remarkably, no shift of the absorption maximum
was observed in dependence on the pH. This behavior
indicated that drastic changes of the conformation as de-
scribed for flexible polyelectrolytes in the case of a polymer 1
cannot be assumed. In contrast to the absorption, the
fluorescence was strongly pH-dependent. At low pH the

Article
Macromolecules, Vol. 43, No. 1, 2010 471
Figure 4. Optical properties of polymer 1 as a function of the ionic strength: (a) UV-vis spectra; (b) PL spectra (λexc = 370 nm) in water.
known for amphiphilic block copolymers where the thermo-
dynamically controlled formation of micelles was kinetically
locked by very polar solvents.53 Thus, the observed result
was attributed to the amphiphilicity of the polyelectrolyte
brush polymer in context with the unsolubility of the hydro-
phobic compounds in water which immediately led to the
formation of kinetically frozen aggregates.
Conclusion
A new type of amphipilic brush polymer which combines the
properties of a conjugated polymer, a brush polymer, and a
polyelectrolyte is presented. The structural design comprises a π-
conjugated rigid 2,7-polycarbazole backbone substituted with
pH-responsive flexible PLL and branched alkyl side chains.
Typical polyelectrolyte behavior of the PLL side chain within
the polyelectrolyte brush polymer was demonstrated by DLS.
Circular dichroism in liquid and solid-state NMR proved the pH-
dependent secondary structure of the PLL side chains. In liquid,
the secondary structure only consisted of R-helix, while for
measurements in solid state a mixture of R-helix/β-sheet
was found. UV-vis and PL experiments clearly showed that
Figure 5. Tapping-mode AFM height (a) and phase (b) images (scan
size 500 nm ꢀ 500 nm) of spin-coated (U = 2000 rpm, t = 2 min)
polymer 1 on mica from mQ-water as solvent; cpolymer = 0.5 mg/mL.
Vertical scale is 8 nm.
larger particles had a length of about 50-100 nm and the
smaller ones of ∼20-30 nm (Figure 5).
The absence of textured aggregates proposed that no long-
range organization50-52 took place. Such behavior is also

472 Macromolecules, Vol. 43, No. 1, 2010
Fruth et al.
€
conformational changes were restricted to the flexible PLL
side chains while the polymeric backbone remained unaffected.
Preliminary morphology studies demonstrated wormlike struc-
tures of the brush polymer. As these novel amphiphilic brush
polymers are responsive to the ionic strength and pH in solution,
they offer the opportunity to control the self-assembly just by an
external stimulus. While at lower pH the brush polymers are
completely dissolved, they can be aggregated and precipitated by
altering the pH. The polymers will also be of interest as model
compounds for studying the mechanical properties. While typi-
cally stiff polymers have high mechanical strength in the direction
of the main chain, perpendicularly the properties are rather weak.
It is expected that by the change from rod-g-coil to rod-g-rod
polymers the interaction due to an interdigitation of the rigid
combs will be enhanced, and a polymer material with improved
mechanical properties, for example a higher Young modulus, can
be expected.
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